The present invention relates to a fuel injector for an internal combustion engine, in particular a diesel engine, intended in particular to be used in a motor vehicle.
A conventional internal combustion engine comprises at least one cylinder in which a piston slides between two maximum positions. The piston defines with the cylinder and a cylinder head a combustion chamber. In such an internal combustion engine, the purpose of an injector is to supply finely atomized fuel to the combustion chamber of the internal combustion engine.
From the patent FR 2,801,346 on behalf of the applicant, there is known, for example, an injection device for an internal combustion engine comprising a fuel injector 10 such as illustrated in FIG. 6.
This injector 10 comprises a body 12 including a transducer 14 which can generate vibrations in a longitudinal mode at ultrasonic frequencies. The transducer 14 terminates in the lower portion in a nozzle 16 in which the vibrations coming from the transducer 14 are amplified.
The assembly of the transducer 14 has a first inner cavity 18. The first inner cavity 18 is intended to be filled with pressurized fuel. To do this, the first cavity 18 is connected to a fuel supply hole 20 which can be connected to a pressurized fuel supply circuit (not illustrated). The first cavity 18 emerges at the lower end 22 of the nozzle 16, also called the stem of the injector, through an injection hole.
The injector 10 also includes a pin 24, or needle, lying mainly along the axis y-y′. The pin 24 is installed such that it can move axially inside the nozzle 16. The lower end of the needle 24 has a valve head 26 lying outside the nozzle 16. This valve head 26 is designed to come into contact with the inner surface of the nozzle 16 defining the injection hole of the nozzle 16 so as to close the fuel injection hole.
The other end of the pin is provided with a weight 28 connected elastically by a spring 30 to the body 12 of the injector 10. The system 32 composed of the weight 28 and the spring 30 is installed in a second cavity 34 formed in the rear portion of the body 12 of the injector 10.
The pin 24 and spring 30 assembly, which is elastic, exerts an appropriate elastic return force pressing the valve head 26 of the pin 24 on the area of the nozzle 16 surrounding the injection hole. The applied preloading provides on the one hand the sealing of the injection hole made at the end of the nozzle 16 when the injector 10 is supplied with fuel at a given pressure and on the other hand the adjustment for any wear in the area of contact of the valve head 26 of the pin 24 with the nozzle 16.
The weight 28 is fixed, for example, by screwing to the pin 24 so as to create a mechanical impedance break at the interface between the pin 24 and the weight 28.
The value of the weight 28 and the stiffness of the spring 30 are selected to form a system having a very long response time compared with the excitation times of the transducer 14.
The transducer 14 includes an area composed of a stack 36 of active piezoelectric or magnetostrictive components, which, respectively due to the application of an electric or magnetic field, change in thickness.
This stack 36 is clamped between two other elements 37a, 37b composed of an elastic material. The connection between the active components is provided by preloading means such as a nut 38. The stack of several active components adds together the changes in thickness generated by each of the active components, the change in thickness resulting from the total movement of the stack of the active components remaining below the limit of elastic deformation of the preloading means.
Due to the application of an electric voltage to the active piezoelectric elements, these elements deform and produce an elastic deformation which is transmitted to the lower end of the nozzle 16.
Preferably, the assembly 40 composed of the transducer 14 and the nozzle 16 is dimensioned to resonate at the excitation frequency of the active components to amplify the longitudinal movements right to the lower end 22 of the nozzle 16. The pin 24, initially closing the injection hole by means of its valve head 26, deforms due to the pulse which is supplied to it when the nozzle 16 starts to oscillate. This deformation spreads elastically along the whole length of the pin 24 and is reflected at the interface 42 between the pin 24 and the weight 28.
The characteristic responses of the pin 24 on the one hand and the nozzle 16 on the other hand make the end of the pin 24 and the opening oscillate with phase and amplitude variation. This variation results in the opening of an annular slit between the pin 24 and the end 22 of the nozzle 16, the width of the slit depending on the phase difference and the relative difference in amplitude between the oscillation of the end 22 of the nozzle 16 and the oscillation of the valve head 26 of the pin 24.
The minimum opening time of the injector 10 is of the same order as the excitation period applied to the transducer, which excitation can take place at several tens of kilohertz, typically 50 kHz, which authorizes a minimum opening time of the order of 20 μs. This makes it possible to deliver quantities of fuel of the order of one microliter during a small period of time.
The body 12 of the injector 10 is intended to be fixed to the upper end of the cylinder head of the engine by means which are not illustrated.
Although the injector 10 has indirect means of setting the pin in longitudinal vibration, also known are injectors comprising direct means of setting the pin in cyclic vibration. In particular, an injector is known comprising a stack of piezoelectric ceramics or a magnetostrictive bar mounted directly in the body of the pin and which excites the pin so as to produce elastic deformations of the pin.
In the two types of excitation, direct or indirect, of the pin of the injector, the pin is embedded at one end in a weight. The function of this weight is to create an impedance break so that the deformation waves being propagated in the pin are reflected at the boundary between the pin and the weight.
Moreover, while the injector 10 is of the outward-opening valve type, injectors of the inward-opening valve type are also known. In the case of an injector of the inward-opening valve type, the pin is pressed, at rest, on the inner face of the lower end of the nozzle due to the action of a spring. The spring is mounted in the second cavity. The closing of the injection hole is thus obtained. When the body of the injector is excited, the pin is set in longitudinal vibration. The end of the pin then oscillates between its position for closing the injection hole and a position for opening this injection hole.
It should be noted that, depending on the type of the injector, the spring exerts, on the pin, either a tensile force (in the case of an injector of the outward-opening valve type) or a compressive force (in the case of an injector of the inward-opening valve type).
However, the dimensions of the injector are fixed by the space available on the engine and in the immediate area around the engine. Thus, the volume of the injector being fixed, the space occupied by the weight+spring system providing a large enough impedance break and a satisfactory sealing force at the injection hole may correspond to a spring with a stiffness such that the weight+spring system has a resonance frequency lying in the excitation range fixed by the vibrations of the engine. An excitation of the weight+spring assembly at its resonance frequency causes the injector to open randomly.
A known solution to this problem consists in adding damping means to the weight+spring system. However, this solution only partially solves the problem of the resonance of the weight+spring system, such an arrangement only reducing the amplitude of the oscillations of the weight+spring system excited at its resonance frequency.
It is also known to fix, to the body of the injector, the weight in which the needle is embedded. However, such a solution has the disadvantage that, because of the heating of the injector and therefore the expansion of the body of the injector and the needle, uncontrolled axial forces occur in the needle. These axial forces disturb the cyclic deformation of the needle and therefore the injection by the injector.